Until now, hot reduction processes usually have been conducted by means of electric light arc furnaces of either open or semi-closed construction. These require an extensive exhaust gas purification system, as the non-utilized air quantity taken in by such furnaces constitutes an uncontrollable multiple of the gas produced in the furnace. A sufficiently large exhaust gas purification plant not only increases considerably the cost of the entire plant, but also requires a large area for its construction. In addition, it is difficult to adapt such filtering and purification systems to existing facilities without great expense and/or considerable delay in operation.
In order to decrease the quantity of non-utilized air, and thus be able to utilize smaller purifying plants, it has been suggested to perform the reduction process in closed electric reduction furnaces. Here, the combustion air enters the combustion chamber via a very broad slot, or via several intake openings distributed over the flue gas hood circumference. The combustion air quantity can be adjusted by opening or closing the intake openings. The slide valves or dampers required to alter the openings are controlled depending upon the prevailing temperature and/or flue gas analysis. Nevertheless, it is difficult to sufficiently close an electric reduction furnace with hot and turbulent operation to keep the exhaust gas purification facilities at an economically justifiable size due to the great exhaust gas quantity caused by the amount of non-utilized air entering the intake openings. Therefore, the familiar reduction methods require extremely expensive installations, both in construction and in operation.
In hot reduction processes, there are sometimes extremely hot gas eruptions with temperatures exceeding 2,000.degree. C which penetrate the burden surface, resulting from the formation of sintered bridges on the burden surface. These gas eruptions require that a great distance exist between the flue gas hood ceiling and the burden surface, so that the metallic parts of the ceiling are not touched by the hot gases, but only after they mix with cooler gas. In order to keep the flue gas hood height within justifiable limits, the flue gas hood construction and the conduit elements of electrodes and charge devices must be made of expensive high temperature resistant materials. In addition, water cooling has to be provided, again increasing the cost of the plant.